The term "gel" encompasses wet gels, including hydrogels and alcogels; and gels dried from the wet gels including aerogels and xerogels. The term "aerogel" was coined by S. S. Kistler in U.S. Pat. No. 2,188,007 and is generally utilized to refer to a gel which has been dried under supercritical temperature/pressure conditions. The term "xerogel" is generally utilized to refer to a gel which has been dried by evaporation of the solvent. Gel composition refers to a composition comprising a gel which may further include other components, for example an opacifying agent or coloring agent.
Gel compositions are utilized in a wide variety of applications, including thermal and acoustic insulation; catalyst supports and carriers; filters and molecular sieves; rheology control agents; reinforcing agents; thickeners and electronics; adsorbents; flatting agents; particulate additives; membranes; filters; radiation detectors; coatings; and dielectrics and other applications set forth herein and/or known to those of ordinary skill in the art. Gel compositions having lower rod densities, and/or higher surface areas, and/or higher structure are more advantageous for use in many applications. A gel composition's rod density is related to the gel composition's porosity wherein gel composition's with lower rod densities will generally have higher porosities.
Gel compositions are generally produced by combining a gel precursor and suitable solvent to form a sol and then initiating gelation in the sol to form a "wet" gel comprising the solid gel structure and the liquid solvent. The liquid solvent is then removed to form a dry gel composition.
Aerogels produced utilizing a supercritical drying step generally have lower rod densities than heretofore known gel compositions produced without the use of supercritical drying and have therefore become the gel of choice for many applications. However, the supercritical drying step necessary for the production of an aerogel may require the use of relatively expensive and/or complex processing equipment and conditions and therefore be disadvantageous.
In addition to approaches utilizing supercritical drying, at least several other approaches to producing gels have been proposed.
Alexander et al. U.S. Pat. No. 2,765,242 disclose a process for producing gels utilizing aging in water at high temperature followed by heat treatment in alcohols at temperatures significantly above the boiling point in order to esterify the surface. The gel granules may then be milled until a fine powder is obtained. Disadvantages of the approach disclosed in Alexander et al. include the cost of the high pressure esterification step.
WO 94/25149 discloses a process for the preparation of xerogels by chemical surface modification. The disclosed chemical surface modification agents have the formula R.sub.x MX.sub.y where R is an organic group such as CH.sub.3, C.sub.2 H.sub.5 etc.; X is a halogen and M is Si or Al. Potential disadvantages of the approach disclosed in WO 94/25149 include the high cost of the reagents and potential problems relating to disposal of the by-products of the reaction.
U.S. Pat. No. 5,270,027 discloses a process for preparing silica xerogels using alkanolamines. The disclosed process produces xerogels having a total pore volume variable from 2 to 3 cc/g. The equivalent density of individual granules is 0.29 to 0.37 g/cc. Potential disadvantages of the approach disclosed in U.S. Pat. No. 5,270,027 include the complicated steps disclosed as part of the process, in particular the thermal treatment step, and that the process is not disclosed as producing aerogels with densities low enough for certain applications.